The use of implants in the field of orthopedic surgery has a relatively recent history, even including the use of screws, which can be used to hold plates or rods in position, or to hold bones or bone fragments in place to facilitate fusion between bone surfaces. Achieving repair across a fracture, or fusion between two otherwise unrelated bone segments through the use of a stand-alone screw, is often furthered by causing compression of the segments at the cortical surfaces. The prior art has recognized the use of screws that generate this kind of compression, and includes compression screws that are self-tapping, self-drilling and self countersinking. The present screw is designed to maximize the ease of insertion and the compression generated, in particular to suit the needs of small bone surgeons. This area includes bones distal to the elbow or knee that are smaller more delicate bones than the long bones, and further are bones that function in conjunction with less, and more delicate, soft tissue in order to achieve movements with greater finesse than the gross apendicular movement. There is often little bone for purchase, and even less muscle tissue to provide a buffer for protruding tips. Consequently, it is imperative in the small bone context that implants are designed to maximize the desired result with a minimum of volume. The present invention is designed to provide both a maximum of compression along the longitudinal axis of the screw and to provide for ease of insertion by lessening the need for an additional drilling step.
The present invention can be used for fixation or repair following trauma, for example of the scaphoid bone, or for correction of a developed condition such as bunion or congenital defect like flat feet. Thus, the present invention relates generally to a class of screw known as a “compression screw”, and more specifically to those classed as “headless compression screws” which generally refer in the orthopedic arts to screws which are countersunk so that they that do not project beyond the surface of the bone in use. The present invention has a threaded distal end, which is preferably self-tapping and a proximal end, which has a compression taper or wedge. This area has an increasing diameter toward the distal end, such as is provided, for example, by a frusto-conical or domed shape to the maximum diameter area of the wedge. Beyond the maximum, the proximal portion includes a bevel or rounded edge that terminates in a flat proximal terminal surface that includes a torque driving recess, This proximal taper area has one or more, and preferably two to four, equally spaced cutting flutes, which also end at or below the rounded edge of the wedge. The flute or flutes do not extend through to the proximal terminal surface of the screw, and in fact end substantially before the end, meaning that a circumferential fillet or lip is formed at the top of each cutting flute by that portion of the taper which is proximally distant to the flute. Preferably this feature provides a plurality of lips that act as a stop portion located proximally at or below about the widest portion of the compression wedge. Thus, the proximal compression taper is self-drilling to the bone cortex, but will act to generate compression when the distal side of the lip engages the cortical bone that forms the top of the screw hole. Thus, in one specialized sense this stop portion can be considered to be a “head” in so far as it is not self-countersinking and in that it increases the longitudinal compression generated at the proximal portion of the screw, in particular because of the composition of bone at this surface. It is not intended to sit proud to the bone surface nor to be countersunk, but rather to sit flush with it unless the bone is subjected to a counterboring step prior to implantation. This design helps to provide optimal compression by causing the screw to impact the hard cortical portion of the bone and to thus drive the bone toward the distal end of the screw. Thus, the product is considered to be “self-drilling” but not “self-countersinking. This is an aspect of the invention that distinguishes it from the prior art “headless compression screws”.